Automated cruise control (ACC) for automobiles is gaining popularity in recent years. ACC allows a user to set the desired speed and minimum following distance of his/her vehicle. The system then controls the speed of the user's vehicle to ensure that the minimum following distance is maintained. Critical to such systems is the effective implementation of a radar system, typically those operating in the 77 GHz range. Such systems must be capable of transmitting, receiving and manipulating millimeter-wave (mm-wave) signals. As with most electronics, there is continuous pressure to miniaturize such systems to reduce their space and material requirements. Consequently, the circuitry of these systems is becoming more compact and sophisticated, employing such techniques as stack circuit technology to reduce size. With stacked circuits, there is often a need to transmit a signal between circuit substrates while operating in the mm-wave domain. For example, in ACC system applications, transreceiver and antenna are placed on either sides of a thick support plate. This makes it necessary to transmit the mm-wave signal between two microstrips on either side of the relatively thick metal support plate. This transmission is performed by a “signal transition” or “transition” as used herein. Design of this transition is critical to the overall system performance.
The purpose of a signal transition in an electrical circuit is to transfer the radio frequency (RF) energy from one point to another point with minimum interference and loss. The key requirements of a good signal transition are high return loss and low insertion loss. Note that, in general, these two specifications are independent from each other, but must be satisfied simultaneously. In other words, one may achieve a relatively good return loss using a particular signal transition, however, without having a low insertion loss, mm-wave energy is absorbed in the transition, thereby diminishing the total performance of the system. Having a low insertion loss is especially important in high frequencies due to increased conductor and radiation losses.
Transitions designed to transfer electrical signals from transverse plane of microstrip lines to another plane, which is parallel to the first one, with a vertical connection are now going to be explained in more detail because the invention is related with such structures. Via holes employed in standard multi-layer printed circuit board (PCB) technology are very good examples of such transitions. The critical issue here is the electrical length of the vertical connection. As the length of vertical connection increases, design of the transition becomes more challenging because of the increased parasitic inductance. There are a number of reported developments for transferring a signal from one transverse plane to another one. For example, the microstrip-to-slot transition along with its variants which use a vertical waveguide section is one of the more commonly used techniques for this purpose. This approach, however, has a number of disadvantages. First, this transition relies on the resonance phenomenon to achieve a good match. Therefore it is particularly susceptible to geometry variations in the transition. Additionally, since the transition has no back short, it suffers from relatively high insertion loss due to radiation. This is especially important because the spurious radiations that may occur in such a transition may increase the cross talk or affect the antenna pattern in a mm-wave system. Alternatively, a transition can be used which exploits an E-plane probe with a back short to transfer the energy through a waveguide section. Although this approached is well established in the literature, it has a significant disadvantage in mm-wave frequencies. Specifically, at these frequencies, one must position a back short over a microstrip probe within a tolerance in the order of sub-millimeters in a 77 GHz application. This is clearly an expensive procedure for a high volume manufacturing.
Therefore, there is a need for a mm-wave transition to overcome the aforementioned difficulties. The present invention fulfills this need among others.